The present invention relates to an optical scanning apparatus suitable for use with a laser beam printer or a like device.
FIG. 2 shows a prior art optical scanning apparatus such as the one described in Japanese Patent Publication No. 63-61824. Laser light emitted from a laser oscillator 101 typically using helium or argon as an active material is converged by a lens 102 and sent into an optical modulator 104 via a mirror 103. The optical modulator 104 modulates the laser light in accordance with a recording signal. The modulated laser light is reflected by a mirror 105 and admitted into a collimator lens 106 where it is converted to parallel rays. The parallel rays of light emerging from the collimator lens 106 pass through a slit plate 107 and impinge on a reflecting surface of a rotating polygonal mirror 108. The reflected light passes through an f.theta. lens 109 and illuminates the surface of a drum 110. The f.theta. lens 109 having a focal length f not only converges the collimated light at a point on the drum surface but also insures that the image height y, or the distance from the optical axis of the f.theta. lens to the spot on the drum surface is proportional to the incident angle, .theta., of the beam admitted into the f.theta. lens (y=f.theta.).
Since the polygonal mirror 108 rotates about its axis 111, the surface of the drum 110 is scanned by the spot moving in the main scanning direction (i.e., parallel to the rotating axis 112 of the drum 110). Further, the drum 110 rotates about its axis 112 to cause the spot to scan the drum surface in the sub-scanning direction (i.e., perpendicular to the rotating axis 112). Thus, an image is recorded on the exposure plane of the drum 110 in accordance with the recording signal.
The spot diameter S of light converged by the f.theta. lens is represented by: EQU S=k.lambda.F=k.lambda.f/D (1)
where .lambda. is the wavelength of light, F is an F number which is equal to the focal length f of the f.theta. lens divided by the diameter of incident beam D, and k is a proportionality constant. As shown in FIG. 3, k assumes a greater value when less vignetting occurs in light distribution (FIG. 3a) than it does when extensive vignetting occurs (FIG. 3b).
As is obvious from Equation (1), the spot diameter S is in inverse proportion to the diameter of incident beam D if the focal length of the f.theta. lens is the same. Hence, in order to obtain a beam spot that has a short dimension in the main scanning direction and that has a long dimension in the sub-scanning direction, as shown in FIG. 4, it is necessary to use a slit plate 107 that performs beam shaping in such a way that the cross-sectional shape of the beam is long in the main scanning direction while it is short in the sub-scanning direction. Consequently, the spot formed on the drum 110 is short in the main scanning direction and long in the sub-scanning direction. Thus, if a scanning operation in the main scanning direction is followed by rotation of the drum 110 by a predetermined pitch to perform the next scanning operation, the two regions of scanning will partly overlap to avoid the occurrence of an unscanned region.
However, if the spot shaping slit plate is positioned in the optical path of collimated light, the laser light reflected by the slit plate will return to the laser oscillator 101 to instabilize its operation. Further, the need to provide a separate slit plate increases the number of components and fabrication steps, thus leading to a higher production cost.